Hybrid drives for vehicles are known. In the case of the hybrid drives discussed here, an internal combustion engine is combined with two electric motors, so that a plurality of drive sources is available to the vehicle. In accordance with demands specified by a vehicle driver, the drive sources can selectively supply their input torques to a drive train of the vehicle. In a manner that is known per se, this results in different possible drive configurations depending on actual driving situations, the drive configurations serving in particular to enhance driving comfort and reduce energy use while reducing pollutant emissions.
In the case of hybrid drives for vehicles, serial configurations, parallel configurations and mixed configurations of internal combustion engine and electric motors are known. Depending on the configuration, the electric motors are capable of being engaged directly or indirectly in the drive train of the internal combustion engine. To operatively connect the internal combustion engine and/or the electric motors, it is known to position them in such a manner that they are capable of being operatively connected to each other via gearing, e.g., planetary gearing or the like, and clutches.
To permit optimum implementation of a driver's request for drive power from the hybrid drive, a coordinated actuation of the drive motors of the hybrid drive is required, which takes place in known fashion using an “engine control unit.” The actuation of the drive motors can take place based on a setpoint operating state (optimum operating point) of the hybrid drive to be determined by the engine control unit.
It is known that, to determine the setpoint operating state, the required power of the internal combustion engine, the required speed of a first electric motor and the required torque of a second electric motor must be determined based on a power demand on the hybrid drive, corresponding to a vehicle driver's desired torque, and based on an instantaneous electrical power demand of an electrical system of the vehicle. These determined variables are sent to the drive motors as setpoint variables. It is disadvantageous here that the torque of the second electric motor cannot be adjusted with sufficient accuracy. This instantaneous torque of the electric motor must therefore be estimated based on a measured phase current, for example. Due to this inexact estimation, the adjustment of the torque of the second electric motor is faulty; as a result, the instantaneous output torque of the hybrid drive and the instantaneous power output of the vehicle electrical system deviate from their setpoint values. While the deviation of the output torque can be compensated for by a vehicle driver (by operating the accelerator pedal or the like), the deviation of the output of the vehicle electrical system can only be detected and regulated indirectly. The vehicle system voltage is adjusted by a higher-order controller, which is only capable of operating slowly due to a high capacity of a vehicle battery that is used. This regulation of the vehicle system voltage results in deviations of the instantaneous vehicle system output, making it necessary to correct the setpoint operating point of the hybrid drive.